[core] Animate Overlay2 content alongside backdrop

[dokson]

Summary

Fixes #7781.

Overlay2 wraps backdrop and content in separate CSSTransition components, but only the backdrop had CSS transition rules in _overlay.scss. When closing an Overlay2 with custom content, the backdrop fades smoothly while the content stays fully opaque until the transition timeout fires and it abruptly disappears.

This adds a default opacity fade to .bp6-overlay-content so the content animates in/out together with the backdrop. Dialog and Drawer define their own transitions on the same element (different duration/easing for Dialog, slide-in for Drawer), so they are excluded via :not(.bp6-dialog-container, .bp6-drawer).

Other internal Overlay2 consumers (Popover, Popover-next, OverlayToaster) pass a custom transitionName to Overlay2, so they don't share the bp6-overlay-* enter/exit classes and are unaffected by this change.

Reproduction from the issue: https://codesandbox.io/p/devbox/serene-visvesvaraya-k73346

Test plan

• pnpm --filter @blueprintjs/core run compile (tokens, esm, cjs, esnext, css)
• pnpm --filter @blueprintjs/core run test — 1250 passed
• Inspected compiled CSS to confirm :not() scoping
• Manual smoke check in a browser with bare Overlay2, Dialog, Drawer (deferred to reviewer — local Windows env can't run the docs app reliably)

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Description

[core] Animate Overlay2 content alongside backdrop

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